This is not a joke. When it isn't your money, your incentive is to make a decision that cannot be second-guessed. Imagine you do five projects. Accenture come in and have a big heavyweight process and they follow the process each time, and four of the projects are ok but cost 2x what they could have cost, and the fifth project is late and over budget. Well, the fault clearly lies somewhere other than your decision to hire Accenture, they have a process and a brand name! So you blamestorm a bit and it turns out that there were unknown unknowns that couldn't have been foreseen and so on, and you keep your job.
But if you decide to hire BoutiqueCo and things go wrong, your head is on the block. If you do this five times and four times you save money but the fifth time things blow up, you're fired. BoutiqueCo don't have any credibility when blaming unknown unknowns with their "agile process" and what-not, and nobody remembers the fact that you saved so much money on the first four projects you could fund the fifth and sixth projects with the savings.
Small boutiques cannot get gigs with the companies that can afford to pay the rates of the big firms unless they manage to create a very unique brand or find other markets.